Plasma sources are extensively used to generate photons or ions from sample analytes. In elemental analysis in particular, plasmas are extensively used as analytical tools for environmental monitoring. Due to health concerns, there is a need to improve the capability to monitor the environment. This involves analyzing a larger number of samples which typically occurs in the lab. However, in many cases, such as when accidental spills occur for example, improvements in monitoring the environment can be achieved by obtaining analytical results on-site (i.e. in the field) and in (near) real-time. However, analytical equipment cannot be taken out of the lab for use in the field. Furthermore, analysis of a large number of samples is prohibitive due to the costs involved. In such cases, a large number of samples can only be analyzed if the cost per analysis is reduced, for instance, by reducing the operating cost of the analysis equipment.
Analysis equipment that employs plasma spectrochemistry is useful for environmental monitoring. There are several different types of plasma sources that can be used with this equipment such as inductively or capacitively coupled plasmas, radio frequency or microwave induced plasmas, glow discharges or dielectric barrier discharges, etc. However, with this type of analytical equipment, the plasma source is typically tethered to a wall socket due to the high electrical power requirements of the equipment. Further, the analysis equipment is firmly installed in the lab due its weight, size and gas consumption.
For example, an Inductively Coupled Plasma (ICP) source with a water-cooled load coil is the most widely used plasma source for elemental and isotopic analysis. A pneumatic nebulizer is generally used for introducing liquid samples into the ICP source. For elemental or isotopic analysis, the ICP source requires 1-2 kW of power, and the customarily used ICP torch requires an aggregate gas flow-rate of 12-20 L/min. Due to the relatively large flow-rate and its associated cost, the typical ICP source uses Ar gas which is delivered from bulky and heavy containers, such as compressed-gas cylinders. Additionally, for optimum operation, the pneumatic nebulizer operates continuously using approximately 1-4 mL/min of sample and requiring a carrier-gas with a flow-rate of about 0.7-1.2 L/min. Furthermore, since a nebulizer can only be used with liquids, solids must first be converted to a liquid (e.g. digested using strong acids) prior to their introduction into an ICP source. Because many analytical samples naturally occur as solids, and since acid digestion in the field is unlikely (due in part to safety concerns), use of a nebulizer for sample introduction into portable plasma sources that can be used in the field is unlikely. Furthermore, because there are numerous applications in which very little sample is available for analysis, the requirement for a sample volume on the order of milliliters further restricts nebulizer applicability. This is of particular importance in limited sample-size applications. Additionally, pneumatic nebulizers have low sample introduction efficiency (˜1%), thus degrading detection limits.
Accordingly, there is a need for analysis equipment that has low power and low gas consumption for reducing cost per analysis. Further, there is a need for analysis equipment that is portable so that it can be used in the field. However, for plasma sources to be useful in environmental analysis, there must be a means for introducing analytical samples into them. Accordingly, there is also a need for analysis equipment with an interface or introduction component that can be used to introduce the analytes into the plasma source. One possible solution includes using a miniaturized plasma source with an appropriate interface. The term “miniaturized plasma” will be used in the following discussion as a means of illustration rather than limitation.
Plasma miniaturization is receiving increased attention in the current literature. However, some of the conventional microplasma sources require as much as 200 Watts of electrical power while others require vacuum pumps for proper operation, thus clearly preventing portability. As well, the recent work on conventional microplasma sources do not focus on sample introduction. In fact, most conventional microplasma sources typically simply use gaseous samples because such samples are relatively easy to introduce into microplasma sources. Others required use of high concentrations of a supporting electrolyte (e.g., 0.5 M HNO3 needed to make the pH approximately 1, thus possibly giving rise to contamination and making waste disposal an issue). In addition, they either required a pump to deliver mL/min volumes of sample (thus needing a pump for proper operation) or they required relatively high electrical power levels (e.g., 50-150 Watts) to vaporize the water solvent. Both of these clearly disable portability. In one instance, use of μL volumes of sample and low power (e.g., in the Watts range) operation has been reported. But, there were difficulties with liquid sample introduction and poor sensitivities (in the 10,000 ppm range) were reported. As well as, the device could not be used with gaseous or solid samples. Overall, no microplasma sources have been developed that can be used with all three sample types: namely liquids, solids or gases. Accordingly, conventional microplasma sources restrict analytical capability, utility and applicability by excluding the majority of analytical samples that naturally occur as liquids or solids. For these conventional microplasma sources, it is not clear whether the type of interface used for sample introduction results in the predominant use of gaseous samples.